add a note for empty input (#632)

* add a note for empty input

* address comment

Author: philloooo

index.bs

@@ -1284,6 +1284,10 @@ Create a named {{MLOperand}} based on a descriptor, that can be used as an input
     1. Return |operand|.
 </details>
 
+<div class="note">
+  The {{MLGraphBuilder}} API allows creating an {{MLGraph}} without input operands. If the underlying platform doesn't support that, implementations may add a stub input or passing constants as inputs to the graph.
+</div>
+
 ### constant operands ### {#api-mlgraphbuilder-constant}
 Create a constant {{MLOperand}} that can be used in {{MLGraphBuilder}} methods.